YOLOv3 Synthetic Data Training
I am not absolutely sure, but I guess it is due to the domain gap. As far as I have seen in my project where YOLOv3 was trained on synthetic images, it performed better when the model was trained and tested on the same domain (synthetic), while the performance drops when we introduce real images for testing. So when you include real images along with synthetic images, you might have to use some domain adaptation methods to improve the performance.